The present invention relates to a coating apparatus, and more particularly to a coating chamber for depositing a coating on a surface of an internal air passage of an aerospace component.
A conventional aircraft gas turbine engine generally expands hot combustion gases against shaft-mounted turbine airfoils to rotate the shaft and power a fan and a compressor. A combination of high temperatures and combustion products in the hot combustion gases produces a severe corrosive and oxidative environment for the turbine airfoils. Conventional turbine airfoils employ internal air passages to cool the turbine airfoil. Airflow through the internal air passages carries heat away from the interior of the turbine airfoil. Typical turbine airfoils also employ coatings, such as aluminide coatings, on the surfaces of the internal air passages and exterior of the turbine airfoils to protect the turbine airfoil against corrosion and oxidation that may otherwise decrease the life of the turbine airfoil.
The coatings are typically deposited on the internal surfaces of the internal air passages using a chemical vapor deposition (CVD) process carried out at temperatures on the order of 2000° F. inside a coating chamber. The turbine airfoils are typically each mounted vertically on a hollow mounting boss in the coating chamber, which are welded along with gas supply pipes to walls of the coating chamber. The hollow mounting bosses provide flow paths into to the internal air passages of the turbine airfoils. During the CVD process, a coating chemical flows through the internal passage and coats the internal passage.
Disadvantageously, the coating chamber, mounting bosses, and gas supply pipes may warp after only a few coating cycles because of thermal expansion and contraction, i.e., thermal stress. The warping may cause the turbine airfoils to mount at an angle relative to the vertical position and may disrupt the flow of the coating chemical, thus causing uneven distribution of the coating on the internal air passage surfaces. The warping may also cause leakage of the coating chemical from the hollow mounting bosses. Sealing gaskets have been employed between the turbine airfoil and mounting bosses to counteract the leaking, however, the sealing gaskets are often unreliable and leaking can continue and further contribute to uneven coating distribution. The uneven coating distribution may ultimately result in a low yield of acceptably coated turbine airfoils and/or having to replace the coating chamber.
Accordingly, there is a need for a robust CVD coating chamber that does not warp and avoids the shortcomings and drawbacks of the prior art.